RFR: 8229957: Harden pid verification in attach mechanism

[Leonid Mesnik]

Hi

Could you review following fix which add sanity check of pid value in 
attach mechanism on *nix based platforms.

PID for java process is always positive on affected OS. Hotspot 
internally uses signal (SIGQUIT) while attaching. So using negative 
numbers as pid might cause very unexpected results and should be prevented.

webrev: http://cr.openjdk.java.net/~lmesnik/8229957/webrev.00/

bug: https://bugs.openjdk.java.net/browse/JDK-8229957

I checked that jcmd doesn't allow to connect to negative pids.

Leonid